1. Field of the Invention
This invention relates to an intake system for an internal combustion engine, and more particularly to an intake system for an internal combustion engine in which the engine output power is improved by the kinetic effect of intake air.
2. Description of the Prior Art
As is well known, a negative pressure wave generated in an intake system of an internal combustion engine upon initiation of each intake stroke is propagated upstream of the intake system and is then reflected at an end of the system opening to the atmosphere or to a surge tank disposed on the upstream side of the intake system toward the intake port as a positive pressure wave. By arranging the intake system so that the positive pressure wave reaches the intake port immediately before closure of the intake valve to force intake air into the combustion chamber, the volumetric efficiency can be improved. There have been known various intake systems in which such inertia effect or resonance effect of intake air is used for improving the volumetric efficiency. However, the period of vibration of the pressure wave in the intake passage can be matched with the period of opening and closure of the intake valve to obtain a sufficient inertia effect of the intake air only within a limited engine speed range which depends upon the shape of the intake passage. There has been proposed an intake system in which, for instance, the length of the intake passage is changed according to the engine speed in order to obtain the inertia effect of intake air over a wider engine speed range. For example, in the intake system disclosed in Japanese Utility Model Publication No. 58(1983)-56328 or Japanese Unexamined Patent Publication No. 56(1981)-115819, each of the discrete intake passage portions leading to the respective combustion chambers is bifurcated to form a long passage portion and a short passage portion both opening to a surge tank or the like at the upstream end, and an on-off valve is provided in the short passage portion to open the short passage portion in a high engine speed range to shorten the effective length of the discrete intake passage portion, thereby obtaining a sufficient inertia effect of intake air in the high engine speed range in addition to a low engine speed range.
In the system, the effective length of each discrete passage portion is elongated in the low engine speed range so that the pressure vibration generated in the discrete passage portion contributes to the volumetric efficiency. However, a sufficient effect can be obtained only on the high engine speed side of the low engine speed range.